The freezing and thawing of a liquid flowing within nano-scale capillaries and channels to act as an on/off switch or valve is known in the art, see for example U.S. Pat. Nos. 6,342,184, 6,159,744, 6,007,302 and 5,795,788. This technique uses a flow-switching device, commonly referred to as a “freeze-thaw valve,” to stop or divert liquid flow to a further channel or chamber simply by freezing and thawing the liquid contained within a segment of channel or tubing. This freeze-thaw valve, which allows for the management and control of liquid flowing within channels having a small diameter, does not require any moving parts and does not contribute any unswept dead volume within an analytical system.
Freeze-thaw valves typically comprise a valve body suspended in an insulating housing, wherein the valve body includes an enclosed thermally conductive expansion chamber having a porous metal vent. Flow capillaries are inserted into through-holes in the valve body, where they make intimate thermal contact with the valve body. An electrical resistance heater and thermocouple are attached to the valve body to supply heat to thaw the frozen fluid plug and thus open the valve. These freeze-thaw valves typically operate by projecting a jet of cold gas, such as liquid carbon dioxide and/or liquid nitrogen from a liquefied source of gas under pressure, directly onto a segment of channel or tubing. This causes the liquid flowing within the segment of channel or tubing to freeze, creating a plug of frozen liquid, which blocks the flow of liquid through the valve, i.e. the valve is “closed”. The electrical resistance heater can be energized to produce heat that conducts throughout the valve body and warms the frozen fluid plug allowing the liquid to flow through the valve, i.e. the valve is “open”. Thermocouples or resistance thermal detectors (RTD) may be used to provide temperature sensing for control of the heating and cooling of the valve body.
Known freeze-thaw valves are fabricated using conventional machining techniques in three-dimensional geometry and as a result these valves have been limited to larger sizes. This is undesirable in many applications because the size of the valve affects the valve speed, the refrigerant consumption, the valve reliability and the cost of fabrication. For example, valve performance is related to the thermal characteristics of the valve body, e.g. the valve response time is governed by the thermal diffusion time constant. As such, because larger valve bodies tend to have larger thermal diffusion time constants, temperature changes in the valve body tend to occur more slowly. Thus, freezing and/or thawing a fluid plug takes longer for larger valve bodies than it does for smaller valve bodies. Moreover, because the refrigerant and electrical energy consumption of the valve is governed by the thermal mass of the valve body, larger valve bodies tend to consume greater amounts of refrigerant and electrical energy than do smaller valve bodies. Furthermore, the thermal stresses on the valve are governed by the uniformity of the temperature and similarity of the thermal expansion coefficients of the materials used to fabricate the valve, and therefore the reliability and life cycle of the valve are adversely affected by the valve size and material selection.